1. Field of the Invention
The present invention relates to film forming methods for forming coating films through spin coating of solution on substrates, and particularly relates to film forming methods adapted for forming photoresist films, SOG (Spin On Glass) films, etc. on semiconductor substrates (semiconductor wafers).
2. Description of the Related Art
Hitherto, there is adopted a spin coating method as a method for forming films, such as photoresist films, SOG (Spin On Glass) films, etc., on semiconductor wafers.
According to the spin coating method, a semiconductor wafer is horizontally held and spun, while droplets of solution are dropped on the semiconductor wafer before and after the start of the spin of the semiconductor wafer, whereby a film of the solution is formed on the semiconductor wafer by centrifugal force caused by the spin of the semiconductor wafer. In the formation of films by the spin coating method, films are formed also at peripheral portions of the semiconductor wafer. In the event that the semiconductor wafer, at the peripheral portions of which coating films are formed, is accommodated into, for example, a cassette, a contact of the films formed at the peripheral portions of the semiconductor wafer with the cassette causes dust from the films. Introducing of the dust into a semiconductor device causes a malfunction of the semiconductor device. Thus, after the films are formed on the semiconductor wafer, a solvent is dropped at the peripheral portions of the semiconductor wafer so as to remove the films formed at the peripheral portions of the semiconductor wafer.
FIGS. 5-7 are cross-sectional views each showing a process for forming the SOG film on the semiconductor wafer.
First, as shown in FIG. 5, a droplet of solution 53, in which a material of the SOG film is dissolved, is dropped onto a surface 51a of a semiconductor wafer 51 having a disk configuration, the wafer 51 being fixed on a spin chuck 52 for spinning the semiconductor wafer 51. And thereafter, the semiconductor wafer 51 is spun, for example, at 3000 rpm. When the semiconductor wafer 51 is spun, centrifugal force of the spin causes a coating film 54 by solution 53 to form over a surface 51a to edges 51b of the semiconductor wafer 51 as shown in FIG. 6.
Next, as shown in FIG. 7, a solvent 56 is dropped at a position near inside the periphery of the semiconductor wafer 51, while the semiconductor wafer 51 is kept on spinning.
FIGS. 8 and 9 are illustrations each showing the state in which films formed on the peripheral portions of the semiconductor wafer 51 have been removed by means of dropping solvents as shown in FIG. 7.
In FIG. 7, in the event that the solvent 56 dropped at a position near inside the periphery of the semiconductor wafer 51 has a strong affinity for the coating film 54 covering the surface 51a of the semiconductor wafer 51, or typically the coating film 54 is soluble in the solvent 56, the films formed on the peripheral portions of the semiconductor wafer 51, of the coating film 54 may be removed. However, as shown in FIG. 8, this scheme brings about protuberances of edge portions of a film 57 retained on the surface 51a of the semiconductor wafer 51. Performing a heat treatment for the semiconductor wafer in the state of such protuberances of the edge portions of the film involves such a problem that cracking occurs in the protuberance portions and as a result dust is generated. Further, dusting occurs also in the event such that the semiconductor wafer is mechanically cramped.
For the purpose of preventing occurrence of the protuberances of films, Japanese Patent Application Laid Open Gazette Hei. 9-220505 discloses a method in which in FIG. 7, as the solvent 56 to be dropped, solvents having a weak affinity for the coating film 54 covering the surface 51a of the semiconductor wafer 51, are used. However, in the event that solvents having a weak affinity for the coating film 54 covering the surface 51a of the semiconductor wafer 51, are dropped, as shown in FIG. 9, as to a film 58 of the solution retaining on the surface 51a of the semiconductor wafer 51, an occurrence of protuberances of the peripheral portions of the film 58 are avoided, but a film 59 will be retained at the edges 51b of the semiconductor wafer 51. Accommodating such a semiconductor wafer having the films at its edges into a cassette involves such a problem that dust is generated owing to a contact of the films remaining at the edges of the semiconductor wafer with the cassette at the time of the accommodation of the semiconductor wafer.
In effect, even if solvents having a strong affinity for the solution 53, alternatively solvents having a weak affinity for the solution 53, are dropped onto the semiconductor wafer 51, any of these schemes involves such problems that protuberances of edge portions of the film 57 retained on the surface 51a of the semiconductor wafer 51 occur, or the film 59 is retained at the edges 51b of the semiconductor wafer 51, and thus finally it is impossible to avoid the problem of a generation of dust.
Hereinafter, there will be explained a result of an experiment in which 5 types of solution, A, B, C, D and E were prepared as the solution 53 in which a material of the SOG (Spin On Glass) film is dissolved; 4 types of solvents a, b, c and d were prepared as the solvent 56 for eliminating the peripheral portions of the coating films 54 formed on the semiconductor wafer 51; the coating films 54 were formed on the semiconductor wafers 51 using 5 types of solution, A, B, C, D and E respectively; and the peripheral portions of the coating films 54 were removed using 4 types of solvents a, b, c and d.
Where the 5 types of solution, A, B, C, D and E are Type 7 produced by TOKYO OHKA KOGYOU Co., LTD., Type 2 produced by TOKYO OHKA KOGYOU Co., LTD., FOX-15 produced by Toray Dow Corning Co., LTD., Type 10 produced by TOKYO OHKA KOGYOU Co., LTD. and Type 12 produced by TOKYO OHKA KOGYOU Co., LTD., respectively. The 4 types of solvents a, b, c and d are isopropyl alcohol, cyclohexanone, .gamma.-butyrolactone, MIBK, respectively.
Table 1 shows a result of the experiment mentioned above. FIG. 10 is an explanatory view useful for understanding an evaluation method of three evaluation items (an appearance, a protuberance and an edge bead residual) shown in Table 1.
The "Appearance" shown in Table 1 indicates whether film residuals occur on regions X (shown in FIG. 10) of the semiconductor wafer 51. In case of no film residual, the "Appearance" is denoted by "Good". In the event that film residuals exist, the "Appearance" is denoted by "Residuals Exist".
The "protuberance" indicates a difference H between the thickness of the peripheral portions Y and the thickness of the central portion Z of the film 57 remaining on the surface 51a of the semiconductor wafer 51 when the solvent 56 is dropped on the semiconductor wafer 51.
The "Edge bead residual" indicates whether edge bead residuals occur on the edges 51b of the semiconductor wafer 51. In case of no edge bead residual, the "Edge bead residual" is denoted by "Non". In the event that edge bead residuals exist, the "Edge bead residual" is denoted by the "Present".
What is meant by the "Unavailable" shown in Table 1 is that there is associated with such a problem that dangerous gas for the human body will emanate through a chemical reaction between a film of solution formed by dropping the solution onto the semiconductor wafer 51 and a solvent dropped onto the film of the solution, or a chemical reaction within a liquid tank, and thus the solvent cannot be used for removal of the coating film.
As will be understood from Table 1, even if any of the types of solvents a, b, c and d is applied to the types of solution, A, B, C, D and E, it is impossible to preferably remove the periphery of the coating film 54 formed on the semiconductor wafer 51, prevent protuberances of the film and suppress the edge bead residuals.
In order to resolve the problems of dusting due to the protuberances of the film as mentioned above, Japanese Patent Application Laid Open Gazette Hei. 6-168872 discloses a method in which prior to dropping the SOG solvent onto a semiconductor wafer, the peripheral portions of the semiconductor wafer are treated with HMDS (hexamethyldisilazane) or the like so that the water-repellency can be brought about. According to this method, after the peripheral portions of the semiconductor wafer are subjected to a water-repellent treatment, solution is dropped, and the semiconductor wafer is spun. Thus, the solution, which is spread as a film toward the peripheral portions of the semiconductor wafer by centrifugal force, is shed on the peripheral portions exhibiting the water-repellency, so that the film is formed only at the central area of the semiconductor wafer. In this case, there is a need that the water-repellency is brought about on only the peripheral portions of the semiconductor wafer. However, even if it is intended that the water-repellency is brought about with, for example, HMDS on only the peripheral portions of the semiconductor wafer, the semiconductor wafer is subjected to the HMDS atmosphere in its entirety. For this reason, a film (hereinafter, referred to as a CVD film) produced by, for example, the chemical vapor deposition, is formed on a semiconductor wafer, and after the peripheral portions of the semiconductor wafer on which the CVD film has been formed is treated so that the water-repellency is brought about, solution is dropped to form a coating film. In this case, however, an adhesion of the coating film with the CVD film is extremely degraded. Thus, if a semiconductor device is manufactured in the condition that the adhesion of the films is degraded as mentioned above, then it may happen that a malfunction of the semiconductor device is brought about.
Further, Japanese Patent Application Laid Open Gazette Hei. 8-17817 discloses a method in which after the SOG solution is dropped onto a semiconductor wafer to form a coating film, hydrofluoric acid is dropped at the periphery of the SOG coating film so as to remove the peripheral portions of the coating film. In view of the matter that going of hydrofluoric acid into another film not necessary to be removed causes deterioration of a semiconductor device, and also in view of the fact that hydrofluoric acid is poisonous, it is not preferable that hydrofluoric acid is used.